A synchronous motor has inherent advantages over induction and DC motors. In a synchronous motor, the speed of the rotor is exactly proportional to the frequency of the system which supplies the synchronous motor with electrical power. Accordingly, the synchronous motor is further characterized by the fact that it runs at a constant speed (for constant supply frequency) at leading power factor and has low starting current. The efficiency of such a motor is generally higher than for other electric motor types.
An armature of a synchronous motor is generally built with one set of AC poly-phase distributed windings, usually on the stator or outer annular ring of the motor. As such, the field winding of the motor is usually found on the rotor, and typically consists of more than one pole pair. It is generally understood that the field poles are excited with direct current. The configuration and method of exciting the rotor field windings determines the type of synchronous motor. In general, a separate exciter, slip rings, and brushes are required.
The exciter, slip rings and brushes are eliminated in the synchronous induction motor which has a rotor designed with differing reluctance paths across the air gap separating the stator and rotor to facilitate in developing reluctance torque. There are no field windings on the rotor, and hence no excitation source is required. The stator armature windings are then powered directly from an AC supply line.
The induction motor is simple and cheap to manufacture but does not offer the performance of the synchronous motor. On the other hand, slip-ring synchronous motors are more complex and expensive because of the exciter, slip rings and brushes required for operation. This also increases maintenance requirements, whereas the induction motor is virtually maintenance free. Brush DC motors have similar drawbacks although they are easier to control and have excellent traction characteristics. The slip rings and brushes are eliminated in brushless synchronous motors but the AC brushless exciter configuration still means additional cost, space and complexity.
Permanent magnet synchronous motors appear to be the most attractive. However, the manufacture and high cost of high field strength permanent magnets, and the process of attaching these magnets to the rotor, especially for large machines, becomes an engineering challenge. The maximum field strength of permanent magnets is also limited by the current state of the art. The synchronous induction motor, although very simple in construction, is not very efficient and is generally much larger than a slip-ring synchronous motor for similar performance. In practice, synchronous induction motors have not found much use above a few kilowatts.